The present and continuing increase in data traffic volumes and the requirement for speed of switching and reduced energy consumption in datacenters has driven a great deal of recent innovation. In particular, it has been realized that optical switching offers many of the desired properties but optical devices need to be controlled by and interfaced with electronic devices including traditional electronic data servers.
Optical devices themselves do not necessarily reduce the size or complexity of a switch. In order to improve flexibility in the assembly and application of optical switching units it is desirable to improve the scalability of an optical switch. One way of doing this relates to the topology of the components within the switch network. It is desirable to produce a highly scalable optical switching unit. Thus, there remains a requirement for a packet switch optimally benefiting from the speed of optics and the flexibility of CMOS electronics assembled in an architecture appropriate for huge scalability.
In order to most clearly describe a network topology, for example a computer network, or an optical switching network, as in embodiments of the present invention, the following terminology and notation may be employed:                A graph G is a set of vertices V and a set of edges E, the edges connecting pairs of vertices. The graph may be expressed as G=(V, E). Accordingly, a network may be modelled as a graph, wherein nodes (i.e. individual switching elements) are represented by vertices, and the links between pairs of nodes are the graph edges.        The physical topology of the network is the location in real, 3D space of the nodes and the links.        The logical topology of the network is represented mathematically as the graph of the network G=(V, E).        The radix R of a single switching element is the number of ports on that switching element. The switch ports may be either client ports (connected to external clients such as hosts or servers) or fabric ports (connected to other switching elements), or unconnected.        The number of client ports per switching element=C, and the number of fabric ports per switching element=F.        A path is a sequence of links connecting a source node to a destination node, and the length of a path is the number of links in the sequence. The minimal path between two nodes, is the path with the shortest length, and the diameter of the network is the longest minimal path between any two nodes.        Switching elements in a switch may be arranged into N dimensions (also referred to herein as tiers).        
A known, named network topology is the Folded Clos network. At present, this is a popular topology employed in datacenter networks and multi-chip switches. It is also known as the k-ary n-tree. The network may be described in terms only of R and N:
                    C        total            =              total        ⁢                                  ⁢        number        ⁢                                  ⁢        of        ⁢                                  ⁢        client        ⁢                                  ⁢        ports              ,                            i          .          e          .                                          ⁢          the                ⁢                                  ⁢        number        ⁢                                  ⁢        of        ⁢                                  ⁢        clients        ⁢                                  ⁢        which        ⁢                                  ⁢        may        ⁢                                  ⁢        be        ⁢                                  ⁢        interconnected            =                        2          ⁢                                                    (                                  R                  2                                )                            N                        .                                                  ⁢            P                          =                              total            ⁢                                                  ⁢            number            ⁢                                                  ⁢            of            ⁢                                                  ⁢            switching            ⁢                                                  ⁢            elements                    =                                    (                                                2                  ⁢                                                                          ⁢                  N                                -                1                            )                        ⁢                                          (                                  R                  2                                )                                            N                -                1                                                              Diameter    ,          D      =              2        ⁢                  (                      N            -            1                    )                    
Table 1 below shows the value of N, the number of client ports for various different values of the parameters, which indicates, as discussed above, the number of external clients which may be connected using this network with the given parameters.
TABLE 1Values of N for a Folded Clos network having varying values of R and N.CtotalN = 2N = 3N = 4N = 5R = 48163264R = 8321285122,048R = 12724322,59215,552R = 161281,0248,19265,536R = 242883,45641,472497,664R = 325128,192131,0722,097,152R = 642,04865,5352,097,15267,108,864R = 1288,192524,28833,554,4322,147,483,648
FIGS. 14A to C show examples of Folded Clos network arrangements having different values of N and R, along with their corresponding values of Ctotal, P and D. Clearly, when used in real switching networks the number of switching elements employed is vastly higher than the number shown in these drawings. In these examples, the client ports are represented by the unconnected links on the bottom row of switching elements in each case. Herein, the term “leaves” may be used for switching elements which are connected to both clients and other switching elements, and “spines” may be used for those switches which connect only to other leaves. Throughout this application, the terms “spine” and “active switch” may also be used interchangeably.